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In cricket, one bowler will bowl from one end of the pitch for an over. When this is finished, a different bowler must bowl the next over from the other end of the pitch.

An over is completed when the bowler has bowled 6 legal deliveries (note that in the past an over used to consist of 8 deliveries). If the bowler bowls a wide or a no-ball this does not count as a legal delivery (although any runs scored do count) and the bowler must bowl an extra delivery to make up for it. In theory this means an over could continue indefinitely if the bowler continued to bowl wides and no-balls, it is not unheard of for a bowler to end up bowling 10 or even more balls in a single over due to repeated wides or no-balls.

For completeness, a wide is defined as a delivery that is too far either to the side or over the top of the batsmen. The exact rules as to what constitutes a wide varies between Test match and limited overs cricket, with the latter being more strict. A wide counts as one run to the batting team in addition to any other runs scored. A no-ball most commonly occurs if the bowler oversteps the crease when bowling. The bowler has to have a least some part of their front foot behind the bowling crease as they deliver the ball. A no-ball scores 1 run to the batting team in addition to any other runs scored and as an extra bonus, the batsmen cannot get out off a no-ball by any means other than a run-out (or certain other rare forms of dismissal, such as "obstructing the field").

In cricket, an over is a set of six balls bowled by a player called bowler from one end of a cricket pitch.

In a normal over, a single bowler delivers six balls in succession,
from one end of the pitch, to the batsman at the other end. After six
deliveries, the umpire calls 'over'; the fielding team switches ends,
and a different bowler is selected to bowl an over from the opposite
end of the pitch, while the batsmen do not change ends. The captain of
the fielding team decides which bowler will bowl any given over, and
no bowler may bowl two overs in succession.

An over must consist of six legal deliveries. If the bowler bowls a
wide or a no ball, that illegal delivery is not counted
towards the six-ball tally, and another delivery will need to be
bowled in its place.

There were eight ball overs in Australia for a long time, but they went to six sometime last century (yes, rather vague I know). South Africa had eight at one time as well, I learned this recently while watching an SA test match.

It all seems to be six now, in the full 11-player forms of the game anyway.
The only modern variations I have ever seen have always been when Billy Bowden was standing at the bowler's end.